Optical information recording medium and production method of the same

ABSTRACT

In a single-sided multi-layer optical disk, “n” information layers are formed so that sector address portions ( 9, 11 ) do not overlap with each other in a stack direction in a combination of adjacent information layers ( 1, 4 ) and an optical separating layer ( 3 ) is disposed between the information layers. Thereby, signals of the sector address portion can be reproduced stably without being affected by the sector address portions of the adjacent information layer(s).

TECHNICAL FIELD

The present invention is related to an optical information recordingmedium on and from which large volume data is recorded and reproducedusing a laser beam and a method for producing the optical informationrecording medium.

BACKGROUND OF THE INVENTION

There is a phase-change type optical disk as an optical informationrecording medium in or from which a signal is recorded and reproduced bya laser beam. In the phase-change type optical disk in and from whichthe signal is repeatedly recorded and reproduced, a chalcogenide isgenerally employed as a material for a recording layer.

Further, an optical disk of a single-sided dual-layer structure isproposed for the purpose of increasing recording capacity of a writableoptical disk or a writable and erasable (that is, an overwritable)phase-change type optical disk (see, for example, Japanese Patent Kokai(Laid-Open) Publication No. 2000-36130).

Signals may be recorded on the optical disk according to a formatwherein a sector structure having sector address portions is employed ora continuous recording format. In the optical disk of the sectorstructure, an area for managing an information signal to be recorded anda data area wherein the information signal is recorded by a user areseparated.

In the case where the sector structure is applied to a single-sidedmulti-layer medium having two or more recording layers, there is aproblem that a reproduced signal from an information layer is deformeddepending on the recorded state of another information layer adjacent tothe information layer. In other words, the signal amplitude and thesignal level of the reproduced signal depends on whether a signal isrecorded on the adjacent recording layer(s). In addition, there is alsoa problem that the reproduced waveform is deformed at a boarder betweenthe sector address portion and the data area of the adjacent informationlayer(s) since the sector address portion is always unrecorded and thesignal is recorded only on the data area. However, in practical use,error correction allows the information recorded on the data area to bereproduced without being problematically affected by the sector addressportion of the adjacent information layer(s). On the other hand, sincethe error correction for the reproduction of the information from thesector address portion is weak, the reproduced information is directlyaffected by the adjacent sector address portion. That is, in thesingle-sided multi-layer medium, the problem tends to be caused in thepractical use due to the error of the reproduction of the informationfrom the address portion.

For the purpose of resolving this problem, there is disclosed a solutionwherein the sector address portions of each information layer arecompletely overlapped in a stacked direction and an amount ofdislocation between the sector address portions is detected andcorrected even if the overlapped sector address portions are dislocated(see, for example, WO00/23990). However, the complete overlap of thesector address portions is more time-consuming and requires more costsupon producing the optical information recording medium and it does notgive a substantial resolution to the problem that the peripheralboundary region of the sector address portion affects the adjacentinformation layer(s) (especially the sector address portion(s)). On theother hand, the detection and correction of the amount of dislocationbetween the section address portions requires a complicated recordingand reproducing apparatus, resulting in another problem of cost rise.

DISCLOSURE OF INVENTION

The main object of the present invention is to provide an opticalinformation recording medium of a single-sided multi-layer structurewhich resolves the problem, that is an optical information recordingmedium from which information of an sector address portion is reproducedwithout being affected by the adjacent information layer(s) and toprovide a method for producing such an optical information recordingmedium.

The present invention provides a single-sided multi-layer opticalinformation recording medium including a substrate and “n” (n≧2)information layers which are formed on the substrate and on and fromwhich a signal can be recorded and reproduced by a laser beam that isapplied through the substrate, wherein an optical separating layer isformed between the information layers, each of the information layershas a sector structure having sector address portions and data areas forrecording information signals, the sector address portion and the dataarea are divided in a circumferential direction, and the sector addressportions of each information layer do not overlap with at least thesector address portions of the adjacent information layer(s) in adirection of stack of information layers.

The “information layer” refers to a layer which includes at least arecording layer where a recorded mark is formed by irradiation of alaser beam. The “optical separating layer” refers to a layer which isprovided in order to separate the information layers with a distancetherebetween so that when the information signal is recorded on orreproduced from any one of “n” information layers, the laser beam is notfocused on another information layer. That is, the optical separatinglayer is a layer which is provided so that the signal is not recorded onor reproduced from two or more information layers by the laser beam atthe same time upon recording the signal on or reproducing the signalfrom each information layer.

Herein, “the sector address portions of each information layer do notoverlap with at least the sector address portions of the adjacentinformation layer(s)” means that when a section is viewed taken alongone sector address portion in a direction of stack of information layers(that is, a thickness direction of the medium), no sector addressportions (including boundary lines thereof) are seen in informationlayers that are adjacent above and below to the one sector addressportion. Therefore, when “n”=2, any of the sector address portions inone of the information layers do not overlap with the sector addressportions in the other information layer. When “n”=3, any of the sectoraddress portions in the middle information layer do not overlap with thesector address portions in the upper and lower information layers, butthe sector address portions in the upper information layer may overlapwith the sector address portions in the lower information layer that isnot adjacent to the upper information layer.

This construction enables information on a sector address portion of acertain information layer to be reproduced without being affected byother sector address portions of another information layer(s) that isadjacent to the certain information layer. This reduces the deformationof reproduced signals from the sector address portions, whereby theobject of the present invention can be achieved.

It is preferable that the sector address portions of each informationlayer do not overlap with the sector address portions of any of theother information layers in the direction of stack of informationlayers. That is, when a section is viewed taken along one sector addressportion of the direction of stack of information layers, no sectoraddress portions (including boundary lines thereof) other than the onesector portions are preferably seen in the other information layers.Such a construction reduces deformation of reproduced signals of thesector address portions of a certain information layer by eliminatingthe affect of the sector address portions not only in the adjacentinformation layer(s) but also in other information layers which are farfrom the certain information layer. Assumed that an optical informationrecording medium of this construction is entirely transparent except forthe sector address portions, all the sector address portions can be seenwhen it is viewed from the above side. When “n”=2, this construction isnecessarily obtained. When “n”=3, any of the sector address portions ofthe middle information layer do not overlap with the sector addressportions of the upper and lower information layers, and preferably thesector address portions of the upper information layer do not overlapwith the sector address portions of the lower information layer.

The present invention also provides a method for producing an opticalinformation recording medium including a substrate and “n” (n≧2)information layers which are formed on the substrate and on and fromwhich layers a signal can be recorded and reproduced by a laser beamthat is applied through the substrate, which method includes:

forming the information layer which has a sector structure having sectoraddress portions and data areas for recording information signals, thesector address portion and the data area being divided in acircumferential direction;

forming an optical separating layer which is to be disposed between theinformation layers; and

positioning the sector address portions of each information layer sothat they do not overlap with at least the sector address portions ofthe information layer(s) that is adjacent to the each information layerin a direction of stack of information layers. An optical recordingmedium produced according to this method is the optical recording mediumof the present invention.

“Positioning the sector address portions of each information layer sothat they do not overlap with at least the sector address portions ofthe information layer(s) that is adjacent to the each information layer”is easier than overlapping the sector address portions completely.Therefore, this invention makes it possible to produce the opticalinformation recording medium which can realize small deformation ofreproduced signals from the sector address portions by the method whichis more easily carried out than the conventional method.

In the production method of the present invention, the information layerhaving the sector structure may be formed by forming a recording layerand other layers which constitute the information layer on a surface ofthe substrate or the optical separating layer which has concaves andconvexities which correspond to the sector structure so that therecording layer and the other layers conform the concaves andconvexities.

Positioning may be previously carried out based on the number of theinformation layers and the sector structure. Alternatively, positioningmay be carried out on site by, for example, rotating the informationlayer to be stacked, looking at the positions of the sector addressportions upon stacking the information layers successively.Alternatively, a preliminary positioning may be carried out followed byfine adjustment of the information layers on site to completepositioning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a structure of an embodiment of anoptical information recording medium of a single-sided dual-layerstructure according to the present invention;

FIG. 2 is a schematic view showing each step of an embodiment of amethod for producing an optical information recording medium of asingle-sided dual-layer structure;

FIG. 3 is a schematic view showing each step of another embodiment of amethod for producing an optical information recording medium of asingle-sided dual-layer structure; and

FIG. 4 is a schematic view showing a more specific structure of a firstand a second information layers of the optical information recordingmedium shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described with reference to thedrawings.

FIG. 1 shows a construction of an embodiment of an optical informationrecording medium (an optical disk) according to the present invention.FIG. 1(a) is a cross-sectional view taken along a radius. As shown inFIG. 1(a), this optical information recording medium includes a firstinformation layer 2 and a second information layer 3 formed on asubstrate 1, an optical separating layer 3 situated between the firstinformation layer 2 and the second information layer 4, and a protectivesubstrate 5. Recording and reproducing information signals may becarried out using a laser beam 7 which is applied from the side of thesubstrate 1 and focused by an objective lens 6. Information is recordedon and reproduced from the first information layer 2 by the laser beam 7which passes through the substrate 1. Information is recorded on andreproduced from the second information layer 4 by the laser beam 7 whichfurther passes through the first information layer 2.

FIGS. 1(b) and 1(c) show the sector structures of the first informationlayer 2 and the second information layer 4 respectively. As shown inFIG. 1(b), the first information layer 2 includes data areas 8 forrecording and reproducing information signals on a surface thereof andsector address portions 9 for managing the location of data to berecorded. The data area 8 has a guide groove for tracking or sample pitsin the form of a spiral. The sector address portion 9 has address pittrains arranged in a pattern corresponding to address information.Generally, the guide groove and the address pits are obtained as aresult of formation of the information layer along convexities andconcavities formed on a surface of the substrate 1 or the opticalseparating layer 3. Therefore, in order to make the dimension, theshape, the number and the location of the guide groove as well as thoseof the address pits as desired, it is necessary to make or select thesubstrate 1 or the optical separating layer 3 which has convexities andconcavities corresponding to the desired guide groove and the addresspits and to form the first information layer on the surface having theconvexities and concavities.

As shown in FIG. 1(c), the second information layer 4 has data areas 10and sector address portions 11 similarly to the first information layer2, and the pattern thereof (that is, the sector location and the numberof sectors) is the same as that of the first information layer 2. Thetwo information layers having the sector structure of the same patternhave the same surface shape (that is, the concavo-convex pattern formedby the pits and the guide groove) viewed from the incidence side of thelaser beam 7. The sector structure of the second information layer 4 isgiven by the convexities and concavities on the surface of theprotective substrate 5 or the optical separating layer 3.

In the optical information recording medium of this embodiment of thepresent invention, the first information layer 2 and the secondinformation layer 4 are disposed so that the circumferential relativepositions of them are totally out of alignment and any of thecircumferential relative positions of sector address portions 9 does notcorrespond to (i.e. does not overlap with) any of the sector addressportions 11, as shown in FIGS. 1(b) and 1(c). As described above, sincethe first and the second information layers 2 and 4 have the same sectoraddress structure, the sector address portions of both informationlayers need to be positioned so that they are completely out ofalignment and then these two information layers need to be laminatedwith the optical separating layer 3 interposed therebetween in order toachieve the illustrated disposition.

Next, a method for producing an optical information recording mediumaccording to the present invention is described. FIG. 2 shows a methodfor producing an optical information recording medium of a single-sideddual-layer structure. In a first film formation step as shown in FIG.2(a), a first information layer 2 is formed on a substrate 1 that hasconcavities and convexities corresponding to a guide groove of a sectorstructure that has sector address portions and data areas divided in acircumferential direction. Similarly, in a second film formation step asshown in FIG. 2(b), a second information layer 4 is formed on a secondsubstrate 5. The second substrate 5 has convexities and concavitiescorresponding to a guide groove of a sector structure that has sectoraddress portions and data areas divided in the circumferential directionsimilarly to the first substrate 1. The second substrate 5 becomes aprotective substrate 5 in a final optical information recording medium.Further, in this production method, the first substrate 1 and the secondsubstrate 5 are bonded together so that the second information layer 4and the first information layer 2 are opposed to each other. For thisreason, the concavities and convexities formed on the substrate 5 arecomplementary to the convexities and concavities formed on the firstsubstrate 1. In other words, in the final optical recording medium, theconcavity which constitutes the guide groove on the first substrate 1overlaps with the concavity which constitutes the guide groove on thesecond substrate 5, whereby both of the first and the second informationlayers 2 and 4 form a groove face G that is nearer to the laser beamwhen viewed from incidence side of the laser beam.

In an application step as shown in FIG. 2(c), an adhesive 101 is appliedon the second information layer 4. In this embodiment, a UV curableresin is employed as the adhesive. The application may be carried outby, for example, a spin coat method. The adhesive 101 becomes an opticalseparating layer 3 in the final optical information recording medium.

In a bonding step as shown in FIG. 2(d), the first information layer 2on the first substrate 1 is opposed to and superposed on the secondinformation layer 4 on the second protective substrate 5 through theadhesive 101. The sector address portions 9 of the first informationlayer 2 and the sector address portions 11 of the second informationlayer 4 may be previously positioned so that the sector address portions9 do not overlap with the sector address portions 11 and then thesuperposition (or the overlapping) of these two layers may be carriedout according to the positioning. More specifically, two informationlayers 1 and 4 may be superposed by moving horizontally one of twosubstrates 1 and 5 after the sector address portions 9 and 11 areopposed so as to satisfy the positional relationship wherein the sectoraddress portions 9 do not overlap with the sector address portions 11.Alternatively, the superposition of two information layers may becarried out viewing the positions of the sector address portions 9 and11, so that the sector address portions 9 do not overlap with the sectoraddress portions 11. In other words, the positioning of the sectoraddress portions 9 and 11 may be conducted on site at the time ofsuperposing two information layers 2 and 4. The superposition on sitemay be carried out by, for example, rotating one or both of the firstsubstrate 1 and the second substrate 5. The sector address portions 9and 11 can be completely out of alignment by rotating one or both of thefirst substrate 1 and the second substrate 5 slightly from the conditionwherein the sector address portions 9 and 11 are almost overlapped inthe direction of stack of information layers.

In any positioning method is employed, the dislocation between thesector address portions 9 and 11 can be confirmed by, for example,examining visually the optical information recording medium wherein thetwo information layers 2 and 4 are superposed, from the incidence sideof the laser beam. Since a light is reflected on the data areadifferently from on the sector address portion, existence of the sectoraddress portions can be easily observed visually. For the illustratedoptical information recording medium of a single-sided dual-layerstructure, it can be confirmed that the sector address portions 9 and 10are out of alignment in the stack direction, if the total number of thesector address portions determined visually is the total number of thesector address portions formed on two information layers 2 and 4.

In the bonding step, rotation or pressurization may be further conductedoptionally, so that the thickness of the adhesiveness between thesubstrates is uniform.

Next, a hardening step as shown in FIG. 2(e) is carried out. Thehardening step is carried out after the bonding step in which twoinformation layers 2 and 4 are superposed in the positional relationshipwherein the sector address portions 9 of the substrate 1 are relativelyand completely dislocated in the circumferential direction from thesector address portions 11 of the substrate 2. In the illustratedembodiment, the adhesive 101 is hardened by applying a light of a UVlamp from the side of the first substrate 1.

By carrying out the above steps, an optical information recording mediumof a single-sided dual-layer structure is obtained wherein dislocationof sector positions between two information layers is generated in thecircumferential direction.

Another method for superposing two information layers formed on thesubstrates is a method wherein a resin sheet of round-shape is employed.In this method, the resin sheet of round-shape is used for separatingthe first information layer from the second information layer so as toproduce an optical information recording medium of a single-sideddual-layer. In this method, a substrate and a protective substrate arebonded to the resin sheet disposing two information layers on bothsurfaces of the resin sheet so that the sector address portions of thefirst information layer are dislocated from the sector address portionsof the second information layer. These two substrates having theinformation layers and the resin sheet are bonded together using apressure-sensitive adhesive or a UV curable resin. More specifically,one of the substrates is bonded to the resin sheet and then the othersubstrate is bonded to the resin sheet by following the result of thepreliminary positioning and/or by positioning on site so that twoinformation layers are stacked.

The optical information recording medium of a single-sided dual-layerstructure produced according to any one of the methods described abovehas an advantage that the signals obtained upon reproducing the sectoraddress portions 9 have small or no deformation caused by the effect ofthe sector address portions of the information layer(s) adjacent to thesector address portions 9. Therefore, the signals recorded on thismedium have a good reproducing characteristic and detection error isreduced or eliminated.

Next, a method wherein an optical separating layer is formed by a 2P(photo-polymerization) method is described as another embodiment of theproduction method of the present invention. In this production method,the sector structure of the second information layer is formed byimparting concavities and convexities to the surface of the opticalseparating layer using a stamper and forming the second informationlayer on the concavo-convex surface.

FIG. 3 shows a production method of the optical information recordingmedium of a single-sided dual-layer structure, which includes a step offorming the optical separating layer by the 2P method. In the step shownin FIG. 3(a), a first information layer 2 is formed on a substrate 1having a guide groove of a sector structure including sector addressportions and data areas. This step is the same as that shown in FIG.2(a).

The step shown in FIG. 3(b) is a step wherein a transparent resin 112that is to become the optical separating layer 3 is applied to thestamper 111 having convexities and concavities on its surface. Theconcavities and convexities on the surface of the stamper 111 are formeddepending on the sector structure that is to be formed in the secondinformation layer. The transparent resin layer 112 may be, for example,a UV-curable resin.

The step shown in FIG. 3(c) is a step of bonding the first substrate 1having the first information layer 2 to the resin layer 112, the layer 2being opposed to the stamper 111. This step may be carried out accordingto a preliminary positioning which has been made so that the sectoraddress portions of one information layer does not overlap with those ofthe other information layer. In this method, the positions of the sectoraddress portions of the second information layer are determined based onthe convexities and the concavities of the stamper. More specifically,the bonding step may be carried out by moving horizontally one of thestamper 111 and the first substrate 1 having the first information layer2 formed thereon after they have been opposed to each other so as tosatisfy the positional relationship wherein the sector address portionsof one information layer does not overlap with those of the otherinformation layer. Alternatively, the positioning may be carried out onsite by rotating the stamper 111 and/or the first substrate 1. Further,in this step, rotation and/or pressurization may be optionally conductedso that the distance between the first substrate 1 and the stamper 111is uniform. Next, the resin layer 112 is hardened by applying anultraviolet light from the side of the first substrate 1.

The step shown in FIG. 3(d) is a step of removing the first substrate 1from the stamper 111 at the boundary between the stamper 111 and theresin layer 112. After these steps, the optical separating layer 3having the concavities and convexities on its surface is formed on thesurface of the first information layer 2.

The step shown in FIG. 3(e) is a step of forming a second informationlayer 4 on the surface of the optical separating layer 3. After thisstep, a protective substrate 5 is stacked on the second informationlayer 4.

As a variant of the production method shown in FIG. 3, there is a methodwherein the second information layer 4 is firstly formed on a secondsubstrate 2 which is to become the protective substrate 5 and then theoptical separating layer 3 is formed on the second information layer 4.This production method makes it possible to finally form the firstsubstrate 1 on the first information layer 2 as a thin film by, forexample, a spin coat method. The optical information recording mediumwherein the thickness of the first substrate 1 is small is suitable forrecording and reproducing information with a laser beam of a shortwavelength.

In the optical information recording medium of a single-sided dual-layerstructure, each information layer includes, as a recording layer, a thinfilm whose optical characteristic is changed by absorbing a focusedlaser beam, the optically changed phase being able to be distinguishedby the laser beam 7. The recorded information signals in the recordinglayer of each information layer is reproduced by applying the laser beam7 to the first and the second information layers 2 and 4 and detectingchange in intensity of reflected light. Upon the reproduction, it isimportant that the applied laser beam 7 is exactly focused on theinformation layer to be reproduced. Particularly, the first informationlayer 2 preferably has a transmittancy in a range of 30% to 80% withrespect to the laser beam 7 of a wavelength that is employed uponrecording so that a light beam having a sufficient intensity can reachthe second information layer 4.

The information is recorded on and reproduced from the secondinformation layer 4 by the laser beam 7 which has passed through thefirst information layer 2. For this reason, the recording layerconstituting the second information layer 4 preferably has a highabsorbance with respect to the laser light of a wavelength employed forrecordation and has a high reflectance with respect to the laser beam 7of a wavelength employed for reproduction.

Each element constituting an optical information recording medium of thepresent invention is more specifically described.

FIG. 4 shows a construction of a first information layer 2 and a secondinformation layer 4 of an optical information recording medium of thepresent invention in detail. In the medium shown in FIG. 4, the firstinformation layer 2 includes a first recording layer 23 and twoprotective layers 21 and 25 which protect both surfaces of the layer 23.The second information layer includes a second recording layer 43, twoprotective layers 41 and 45 which protect both surfaces of the layer 43and a reflective layer 47. These two information layers 2 and 4 areseparated by an optical separating layer 3. In the illustratedembodiment, each of two information layers has a guide groove. In FIG.4, a face which is nearer to the laser beam 7 is shown as a groove face“G.”

Firstly, a substrate 1 and a protective substrate 5 are described. Thesubstrate 1 is made of a material which is transparent to a wavelengthof an applied laser beam. Such materials include a resin such as apolycarbonate and a PMMA, and a glass material. In the surface of thesubstrate 1 on which the first information layer is formed, address pitsconstituting the sector address portions 9 are formed and concavitiesand convexities corresponding to a guide groove as illustrated in FIG. 4may be formed if necessary. The guide groove is a continuous grooveguiding the laser beam 7 and it may be referred to as a “track.” Thesubstrate having the convexities and the concavities may be formed by,for example, applying a mastering process which is employed in a compactdisk (CD) and a digital versatile disk (DVD).

The thickness of the substrate 1 may be generally in a range of 0.5 mmto 0.7 mm. When the optical separating layer 3 is formed by the 2Pmethod on the second information layer 4 formed on the second substrate5, the substrate 1 may be a thin substrate formed by a spin coat method.

The protective substrate 5 may be preferably formed of the same materialas that of the substrate 1 and has the same thickness of that of thesubstrate 1 in order to suppress the curvature of the entire opticalinformation recording medium. The substrate 5, however, does not need tobe transparent with respect to a wavelength of the applied laser beam.Also the substrate 5 may be formed by applying a mastering process ormay be formed by the spin coat method when the optical separating layeris formed by the 2P method, similarly to the substrate 1.

It is preferable that the materials which constitute the protectivelayer 21, 25, 41 and 45 are physically and chemically stable. In otherwords, it is preferable that the materials have melting points andsoftening temperatures higher than melting points of the materialsconstituting the first recording layer 23 and the second recording layer43 and they do not present solid solubility in the material of therecording layer. Further, the protective layers may be preferablytransparent with respect to a wavelength of the laser beam. Thematerials constituting the protective layer includes a material selectedfrom dielectrics such as Al₂O₃, SiO_(X), Ta₂O₅, MoO₃, WO₃, ZrO₂, ZnS,AlN_(X), BN, SiN_(X), TiN, ZrN, PbF₂ and MgF₂, or any combinationthereof. When the protective layer is formed of the dielectric, theprotective layer may be referred to as a “dielectric layer.” Theprotective layer, however, does not need to be made of the electric orthe transparent material, and it may be formed of a material such asZnTe which has an optical absorbency with respect to a visible light andan infrared ray. At least one of the illustrated four protective layers21, 25, 41 and 45 may be formed of a material different from materialsof the other protective layers. For example, these four protectivelayers may be formed of materials different from each other. In thatcase, there is an advantage that the freedom of disk design in opticaland thermal terms is increased. However, four protective layers may beformed of the same material. The protective layer may be formed by avapor deposition method using an electronic beam, a sputtering method,an ion plating method, a CVD method, or a laser sputtering method.

The thickness of each protective layer may be selected depending on thewavelength(s) of the laser beam which is employed for recordation andreproduction. The thickness of each protective layer may be generally ina range of 20 nm to 200 nm. The protective layers disposed above orbelow one recording layer are not necessarily of the same thickness, andone of them may be a thin layer while the other may be a thick layer.

The first recording layer 23 and the second recording layer 43 arelayers wherein phase change is caused by irradiation of the laser beamand recorded marks are formed. When the phase change is reversible,erasing and rewriting can be carried out. The phase change generallyoccurs between a crystal phase and an amorphous phase. The phase changemay occur between a crystal phase and a crystal phase. The phase changematerial constituting the recording layer may be, for example, amaterial whose main component is Te, In or Se. More specifically, thephase change materials include Te-Sb-Ge, Te-Ge, Te-Ge-Sn, Te-Ge-Sn-Au,Sb-Se, Sb-Te, Sb-Se-Te, In-Te, In-Se, In-Se-Tl, In-Sb, In-Sb-Se andIn-Se-Te. An experiment which is aimed at finding a material whoseoverwrite cycle-ability is excellent and determining its materialcomposition shows that a three-element based composition whose maincomponents are Ge, Sb and Te is preferable. When the atomic ratios ofthese atoms are represented by Ge_(x)Sb_(y)Te_(z), a compositionrepresented by 0.10≦x≦0.35, 0.10≦y, and 0.45≦z≦ 0.65 (herein x+y+z=1) isparticularly excellent. The thickness of each recording layer may be,for example, in a range of 10 nm to 15 nm. The recording layer may beformed by a deposition method using an electron beam, a sputteringmethod, an ion plating method, a CVD method, or a laser sputteringmethod.

The material of the first recording layer 23 may be the same as ordifferent from the material of the second recording layer 43. Further,the thicknesses of two recording layers may be different from eachother. In any case, the material and/or the thickness of the firstrecording layer 23 are appropriately selected so that the entire firstinformation layer 2 has the transmittancy as described above.

The recording layer formed of a material whose phase changes between thecrystal phase and the amorphous phase is generally formed into a film inan amorphous state and may be subjected to an initialization process ifnecessary. The initialization process is a process in which thetemperature of the recording layer in the amorphous state is raised to atemperature more than the crystallization temperature so as tocrystallize the layer.

The optical separating layer 3 is an intermediate layer disposed betweenthe first information layer 2 and the second information layer 4. Thefunction of the optical separating layer is as described above. Thethickness of the optical separating layer 3 may be generally in a rangeof 10 μm to 100 μm, and preferably in a range of 30 μm to 60 μm. Theoptical separating layer 3 is formed of a material transparent withrespect to a wavelength of the laser beam which is applied to recordsignals on the second information layer 4 and to reproduce the recordedsignals. This is because a sufficient amount of light is secured at thesecond information layer 4. The optical separating layer 3 may be formedof, for example, an epoxy-based UV-curable resin. When the opticalseparating layer 3 is formed by bonding a resin sheet, a two-sided tapefor optical disk bonding (for example, a pressure-sensitive adhesivesheet DA-8320 manufactured by NITTO DENKO CORPORATION).

The reflective layer 47 may be made of a metal element selected from Au,Al, Ni, Fe and Cr, or an alloy thereof. The reflective layer 47 ispreferably formed since it serves to enhance the optical absorptionefficiency of the second recording layer 43. The thickness of thereflective layer 47 may be generally in a range of 50 nm to 180 nm. Thereflective layer 47 may be formed by a vapor deposition method using anelectron beam, a sputtering method, an ion plating method, a CVD method,or a laser sputtering method.

The reflective layer 47 is formed on a surface of a substrate that is tobecome the protective substrate 5 when the optical information recordingmedium is produced by bonding two information layers formed on twosubstrates together. In that case, on a surface of this reflectivelayer, the protective layer 45, the recording layer 43 and theprotective layer 41 which constitute the second information layer areformed in this order. As shown in FIG. 3, when the optical separatinglayer 3 is formed by the 2P method, the reflective layer 47 is formed onthe protective layer 45.

In the above, an example of an embodiment of a single-sided dual-layeroptical information recording medium is described. The opticalinformation medium may further include another layer if necessary. Forexample, in an embodiment shown in FIG. 4, an interface layer may beformed between each protective layer and each recording layer. Theinterface layer may be provided so as to prevent mutual elementdiffusion between the protective layer and the recording layer. Theinterface layer may be, for example, a nitride or a carbide, forexample, a material represented by a general formula of X—N or X—O—Nwherein “X” is one element that is preferably selected from Ge, Cr, Si,Al and Te.

Further, a reflective layer may be formed in the first informationlayer. This reflective layer may be formed adjusting its material andits thickness so that the entire first information layer has thetransmittancy as described above.

The optical information recording medium of the present invention is nolimited to the single-sided dual-layer structure wherein “n”=2, and itmay have three or more information layers. When the optical informationrecording medium has three or more information layers, the object of thepresent invention can be achieved by disposing each sector addressportion so as not to overlap with other sector address portions of atleast adjacent information layer(s) in the direction of stack ofinformation layers. The optical information recording medium of thepresent invention having three or more information layers may beproduced by, for example, repeating the step of forming the opticalseparating layer by the 2P method as described above and the step offorming the information layer on the surface of the optical separatinglayer. In that case, each optical separating layer should be formed onthe surface of the information layer by locating the substrate on whichthe information layer is formed and/or the stamper so that the sectoraddress portions of each information layer do not overlap with thesector address portions of the adjacent information layer(s).

The optical information recording medium of the present invention may beconstructed as, for example, a medium on and from which information isrecorded and reproduced using a red laser beam with a wavelength ofabout 660 nm, by appropriately selecting the construction of eachinformation layer, the thickness of the optical separating layer and thethickness of the substrate. Alternatively, the optical informationrecording medium of the present invention may be constructed as, forexample, a medium on and from which information is recorded andreproduced using a blue-violet laser beam with a wavelength of about 405nm. It should be noted that this invention is not limited by thewavelength of the laser beam that is employed for recordation andreproduction and this invention can be applied to any opticalinformation recording medium of a single-sided multi-layer structurehaving a sector structure.

INDUSTRIAL APPLICABILITY

The present invention gives an optical information recording medium of asingle-sided multi-layer structure from which signals of sector addressportions can be reproduced stably, and thus the present invention ispreferably applied to a large-capacity optical disk for recordinglong-time moving image and sound.

1. A single-sided multi-layer optical information recording medium comprising “n” (n≧2) information layers which are formed on a substrate and on and from which a signal can be recorded and reproduced by a laser beam that is applied through the substrate, wherein an optical separating layer is formed between the information layers, each of the “n” information layers has a sector structure having sector address portions and data areas for recording information signals, the sector address portion and the data area are divided in a circumferential direction, and the sector address portions of each information layer do not overlap with at least the sector address portions of the adjacent information layer(s) in a direction of stack of information layers.
 2. The optical information recording medium according to claim 1, wherein the sector address portions of each information layer do not overlap with sector address portions of any other information layers in the direction of stack of information layers.
 3. The optical information recording medium according to claim 1, wherein “n”=2.
 4. A method for producing an optical information recording medium comprising “n” (n≧2) information layers which are formed on a substrate and on and from which layers a signal can be recorded and reproduced by a laser beam that is applied through the substrate, which comprises: forming the information layer which has a sector structure having sector address portions and data areas for recording information signals, the sector address portion and the data area being divided in a circumferential direction; forming an optical separating layer which is to be disposed between the information layers; and positioning the sector address portions of each information layer so that they do not overlap with at least the sector address portions of the information layer(s). that is adjacent to the each information layer in a direction of stack of information layers. 